1. Field of the Invention
The present invention relates to a method and apparatus for transmitting visual information in the form of video signals.
2. Description of the Background Art
A transmission of visual information in terms of video signals such as that from a controller to a distant CRT display unit in a process control system is carried out either by transmitting the video signals from the controller through amplifiers and wave-shaping circuits without modulation, or else by transmitting the video signal with modulation and then demodulating back for display.
In the latter using modulation, the video signals from the controller are sampled at a given sampling frequency, and then the sampled video signals are modulated before further transmission to the CRT display unit.
However, in such a conventional video signal transmission there is a problem of mis-sampling occurring when the difference between a frequency of the incoming video signals and that of the sampling is present, which results in an inaccurate image reproduction at the CRT display unit.
Although this problem of mis-sampling can be ameliorated by providing a higher sampling frequency with respect to an video signal resolution, realization of such a higher sampling frequency calls for an expensive devices using high speed circuit elements, so that it has not been possible to solve the problem at a more practical economic level.